1. Field of the Invention
The present invention relates to a glass antenna disposed on a window glass of vehicles and the like and a method of designing the same.
2. Description of the Related Art
In general, there is widely known a pole antenna having a pole (rod) projected from a vehicle body in an insulated state and power is supplied thereto as an antenna for vehicles. Since the pole antenna is liable to be bent and broken and further produces noise while it travels with a swing of its body, a glass antenna is put into practical use as an antenna in place of it.
As disclosed in Japanese Utility Model Publication No. 63-92409, for example, the glass antenna has an antenna wire disposed in the vicinity of the side portion of a defogger mounted on the window glass of vehicles and electric current is fed to the antenna wire.
The conventional glass antenna, however, has a problem that since the receiving performance of the antenna is tuned by disposing the antenna wire near to the defogger, a qualitative method is not employed to improve the performance of the antenna, tuning is indefinite and difficult to be predicted as well as the arrangement of the antenna itself is complex.
Different from the above glass antenna, as disclosed in Japanese Patent Publication No. 62-131606, there is proposed an antenna composed of a transparent electric conductor film disposed on a glass surface and an antenna body with a current feeding point disposed on the glass surface above the electric conductor film with the antenna body being coupled with the transparent electric conductor film through a capacitor.
According to U.S. Pat. No. 5,029,308, a first antenna conductor extends upward and downward substantially at the center of a defogger region in which defogger heating wires are stretched and the first antenna conductor is electrically connected to the heating wires across it. Further, a second antenna conductor is disposed at the upper portion (or lower portion) of the defogger so that it is coupled to the heating wire at the uppermost portion (or lowermost portion) of the defogger. That is, the first antenna conductor and the second antenna conductor act as a single antenna. When the first antenna conductor is coupled to the second antenna conductor, a direct current flowing to the defogger is divided to the first antenna conductor and a defogging effect is lowered in the vicinity of the above connection. To cope with this problem, the U.S. Pat. No. 5,029,308 discloses a capacitor disposed between the first antenna conductor and the second antenna conductor to prevent the division of the current flow from the defogger to the first antenna conductor. Note, a capacitor having a capacitance which does not have a high impedance (preferably as low as possible) in a receiving frequency band is selected as the capacitor so that the first antenna conductor and the second antenna conductor act as the single antenna.
Japanese Patent Publication No. 55-60304 disposes a first antenna conductor upward and downward in a defogger region and a second antenna conductor outside the defogger region. Then, a first conductor wire and a second conductor wire are disposed on a glass surface in such a manner that the first conductor wire is coupled to the first conductor perpendicularly to it (i.e., parallel with a defogger heating wire) and the second conductor wire is coupled to the second antenna conductor in parallel with the first conductor, and these first and second conductor wires are placed close to each other and connected through capacitive coupling.
The conventional examples proposed above (Japanese Utility Model Publication No. 63-92409 and Japanese Patent Publication No. 62-131606) connect the antenna body to the transparent electric conductor film through capacitive coupling. When a thin electric conductor film is employed to secure the transparency of the electric conductor film to thereby secure the transparency of a glass, however, the electric conductor film cannot help having a high electric resistance value by which the flow of a received current is interfered. Thus, there is a possibility that an excellent performance of the antenna cannot be expected in practical use.
U.S. Pat. No. 5,029,308 is defective in that since the capacitor is selected to have a low impedance in the frequency region of a radio wave to be received, the defogger heating wire acts as an antenna and thus a heating current flowing to the heating wire affects the antenna and eventually the performance of the antenna is deteriorated.
Since Japanese Patent Publication No. 55-60304 does not take the configuration of the antenna disposed outside the defogger into consideration similarly to U.S. Pat. No. 5,029,308, in other words, since it does not prevent the defogger heating wire from acting as an antenna, the performance of the antenna is lowered.
Since these conventional glass antennas intrinsically have an inferior antenna receiving performance, they are required to improve a receiving performance by the addition of an antenna booster for amplifying a voltage induced to the antenna and a matching circuit for converting the impedance of the antennas into the same value of the impedance of a radio receiver when they are put into practical use. Therefore, the amount of manpower necessary to assemble the antenna and a manufacturing cost are increased, and the antenna becomes large and complex in its structure.